Software testing requires understanding of these network knowledge points

1. Network Classification

Telecommunications network (telephone network)

Cable TV Network

Computer network (fastest growing, core technology of the information age)

Here we focus on computer networks

2. Local Area Network and Wide Area Network

1. Local Area Network (Intranet)

Local Area Network (LAN) is a computer network consisting of multiple computers connected in a certain area. "A certain area" refers to the same office, the same building, the same company and the same school. It is usually within a radius of several kilometers. LAN can realize functions such as file management, application software sharing, printer sharing, scanner sharing, workgroup scheduling, email and fax communication services. LAN is closed and can be composed of two computers in an office or thousands of computers in a company.

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2. Wide Area Network (Extranet)

A Wide Area Network (WAN) is a collection of computer networks that span a large, regional area. It usually spans a province, city, or even a country. A WAN includes different subnets of different sizes, which can be either a local area network or a small WAN.

3. OSI seven-layer network model

1) Physical Layer

Activate, maintain, and shut down the mechanical, electrical, functional, and process characteristics between communication endpoints. This layer provides a reliable physical medium for the upper layer protocol to transmit data. Simply put, the physical layer ensures that the original data can be transmitted on various physical media. The physical layer remembers two important device names, repeater (also called amplifier) ​​and hub.

For example, we are more familiar with: network cards, twisted pair cables, cables, etc.

2).Data Link Layer

The data link layer provides services to the network layer based on the services provided by the physical layer. Its most basic service is to reliably transmit data from the network layer to the target network layer of the adjacent node. To achieve this goal, the data link must have a series of corresponding functions, mainly: how to combine data into data blocks, which are called frames in the data link layer. Frames are the transmission units of the data link layer; how to control the transmission of frames on the physical channel, including how to handle transmission errors and how to adjust the transmission rate to match the receiver; and provide management of the establishment, maintenance and release of data link paths between two network entities. The data link layer provides reliable transmission on unreliable physical media. The functions of this layer include: physical address addressing, data framing, flow control, data error detection, retransmission, etc.

Knowledge points about the data link layer: It can be simply understood as the data channel.

1> The data link layer provides reliable data transmission for the network layer;

2> The basic data unit is frame;

3> Main protocol: Ethernet protocol;

4> Two important device names: bridge and switch.

3) Network Layer

The purpose of the network layer is to achieve transparent data transmission between two end systems. Specific functions include addressing and routing, connection establishment, maintenance and termination, etc. The services it provides make it unnecessary for the transport layer to understand the data transmission and switching technology in the network. If you want to remember the network layer in as few words as possible, it is "path selection, routing and logical addressing".

The network layer involves many protocols, including the most important protocol, which is also the core protocol of TCP/IP - IP protocol. IP protocol is very simple and only provides unreliable and connectionless transmission services. The main functions of IP protocol are: connectionless data transmission, datagram routing and error control. Other protocols used in conjunction with IP protocol to realize its functions include Address Resolution Protocol ARP, Reverse Address Resolution Protocol RARP, Internet Message Protocol ICMP and Internet Group Management Protocol IGMP.

The key points about the network layer are:

1> The network layer is responsible for routing data packets between subnets. In addition, the network layer can also implement congestion control, internet interconnection and other functions;

2> The basic data unit is IP datagram;

3> Main protocols included:

IP protocol (Internet Protocol, Internet Protocol);

ICM protocol (Internet Control Message Protocol, Internet Control Message Protocol);

ARP protocol (Address Resolution Protocol, Address Resolution Protocol);

RARP protocol (Reverse Address Resolution Protocol)

4> Important equipment: router.

4) Transport Layer

It is an end-to-end, host-to-host layer. The transport layer is responsible for segmenting the upper layer data and providing end-to-end, reliable or unreliable transmission. In addition, the transport layer also handles end-to-end error control and flow control issues.

The task of the transport layer is to make the best use of network resources according to the characteristics of the communication subnet, to provide the functions of establishing, maintaining and canceling the transmission connection between the session layers of the two end systems, and to be responsible for end-to-end reliable data transmission. At this layer, the protocol data unit of information transmission is called a segment or message.

The network layer simply transmits the data packets sent by the source node to the destination node according to the network address, while the transport layer is responsible for reliably transmitting the data to the corresponding port.

Important points about the transport layer:

1> The transport layer is responsible for segmenting upper layer data and providing end-to-end, reliable or unreliable transmission as well as end-to-end error control and flow control issues;

2> Main protocols included: TCP (Transmission Control Protocol), UDP (User Datagram Protocol);

3> Important equipment: gateway.

4. Common protocols

DHCP protocol

DHCP (Dynamic Host Configuration Protocol) is a network protocol for local area networks. It uses the UDP protocol and has two main uses: to automatically assign IP addresses to internal networks or network service providers, and to users or internal network administrators as a means of central management of all computers.

HTTP

Hyper Text Transfer Protocol (HTTP) is the most widely used network protocol on the Internet. All WWW files must comply with this standard.

What requests does the HTTP protocol include?

GET: Request to read the information identified by the URL.

POST: Add information (such as comments) to the server.

PUT: Stores a document at the given URL.

DELETE: Delete the resource identified by the given URL.

TCP/IP

The TCP/IP protocol is the most basic protocol of the Internet and the foundation of the Internet international network. It consists of the IP protocol of the network layer and the TCP protocol of the transport layer. In layman's terms: TCP is responsible for discovering transmission problems. Once there is a problem, it sends a signal and requires retransmission until all data is safely and correctly transmitted to the destination. IP specifies an address for each Internet device.

The IP layer receives data packets from the lower layer (network interface layer such as Ethernet device driver) and sends the data packets to the higher layer---TCP or UDP layer; conversely, the IP layer also transmits data packets received from the TCP or UDP layer to the lower layer. IP data packets are unreliable because IP does not do anything to confirm whether the data packets are sent in sequence or have not been damaged. IP data packets contain the address of the host that sent it (source address) and the address of the host that received it (destination address).

TCP is a connection-oriented communication protocol. A connection is established through a three-way handshake. When the communication is completed, the connection is torn down. Since TCP is connection-oriented, it can only be used for end-to-end communication.

The protocols using UDP include: TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), DNS (Domain Name Resolution Protocol), NFS, and BOOTP.

The difference between TCP and UDP:

TCP is a connection-oriented, reliable byte stream service;

UDP is a connectionless, unreliable datagram service.

TCP is reliable, ensuring data integrity, retransmission, and error control.

UDP is unreliable.

How to choose:

High security level: TCP

Small amount of information and unimportant: choose UDP